Lithium batteries, also called lithium secondary batteries, are batteries that contain liquid, gel-like polymer, solid polymer, or polymer electrolyte and generate a current through the movement of lithium ions, and contain cathode/anode active materials comprising polymers. The lithium secondary batteries are composed of a cathode current collector (aluminum, nickel), a cathode active material layer (polymer cathode material such as metal oxide, carbon black, metal sulfide, electrolyte solution, or polyacrylonitrile), an electrolyte layer (carbonate electrolyte solution such as propylene carbonate, ethylene carbonate, dimethyl carbonate, or ethylene methyl carbonate, inorganic solid electrolyte composed of lithium salts, gel electrolyte), an anode active material layer (polymer anode material such as lithium metal, alloy, carbon, electrolyte solution, or polyacrylonitrile), an anode current collector (copper, nickel, stainless), and a casing for packaging them. The lithium batteries, because of their high volume efficiency and weight efficiency, have recently been used in various fields as a small high-capacity power source for, for example, personal computers, portable terminals (e.g., cellular phones, PDAs), video cameras, electric vehicles, rechargeable batteries for energy storage, robots, and satellites.
Casings of lithium batteries include metal cans produced by press-working metal into a cylindrical or rectangular parallelepiped container, and bags made of a multilayer film composed of outermost layer/aluminum/sealant layer. The metal cans, however, have a rigid outer wall and thus define the shape of batteries and, since hardware is designed for a battery, the dimensions of the hardware containing the battery is determined by the battery. Due to such a problem of limitation on design, bag-shaped casings made of a multilayer film are becoming preferred. Physical properties and functions required for casings of lithium batteries include moisture resistance, content resistance (resistance to electrolyte solution or other compounds to be contained, i.e., stability to contents), and moldability and, for example, polyamide films are now used as a film material satisfying such physical properties and functions (see, for example, JP 2006-236938 A). However, the polyamide film, which is insufficient in moisture resistance and stability to contents, may be deteriorated when the contents adhere to the film during processing, and there has been a need for improvement. In addition, the use of polyester films are also being investigated (see, for example, JP 2004-362953 A). However, polyester films that have been proposed are not necessarily sufficient in moldability and difficult to deep-draw. A polyester film with improved deep drawability is also proposed (see, for example, JP 2011-204674 A).
Also for pharmaceutical packaging, there is a growing need for a packaging form containing metal foil such as aluminum foil, to prevent degradation of contents, and metal foil with improved moldability to conform to the shape of contents is demanded.
The polyester films described above have improved deep drawability, but have not simultaneously solved the problem of warping of a molded product toward the polyester film side after molding a laminate obtained by laminating metal foil and the problem of occurrence of delamination between the polyester film and the metal foil.